Strong in Vitro Cytotoxic Potential of New Ruthenium–Cymene Complexes

Two p-cymenerutheniumchlorido complexes with thiourea derivative of 7-chloroquinoline (C1) and pyridine-3-imidazole (C2) were synthesized starting from [(η6-p-cymene)­RuCl2]2 and corresponding ligands. The structures of complexes were determined with elemental analysis and IR, ESIMS, 1H and 13C­{1H} NMR, and 2D 1H–15N correlation NMR spectroscopy. Cytotoxic activities examined by the MTT assay were performed in five human neoplastic cell lines (HeLa, K562, A549, MDA-MB-231, EA.hy926) and one nontumor human fetal lung fibroblast cell line (MRC-5). Tested complexes exhibited low micromolar activities with IC50 in the range 11.03–56.45 μM, while ligands L1 and L2 were significantly less active. Complex C1 showed cytoselective activity toward the K562 cell line (IC50 = 11.03 ± 1.39 μM) and was 3 times less active against the nontumor MRC-5 cell line. Flow cytometry analysis indicated that complexes C1 and C2 after 24 h treatment caused a concentration-dependent increase of the apoptotic sub-G1 fraction (up to 18.4%), comparable to cis-diamminedichloridoplatinum­(II) (cisplatin, CDDP), although without other substantial alterations of the cell cycle. A drug-accumulation and DNA-binding study performed by ICP-MS in the K562 cell line revealed that complex C1 had a high intracellular uptake (1.38 μg Ru/106 cells), which significantly exceeded the intracellular uptake levels of CDDP (0.29 μg Pt/106 cells) and C2 (0.08 μg Ru/106 cells). However, both ruthenium complexes C1 and C2 bind to cellular DNA less efficiently in comparison to CDDP. The structure–activity relationship clearly suggested that introduction of a 7-chloroquinoline moiety in the ruthenium­(II)-p-cymene complex significantly contributed to the intracellular uptake of C1 and higher cytotoxicity and cytoselectivity.